I have had zero experience with these types of soils as well... Obviously. However, it seems to me that run off, even just a little bit to get a pH reading, would be beneficial. That way I know the soil is truly balanced. Not to mention the PPMs in and out.
With that in mind and what you advise, which agrees with many others opinions, I think will kill both birds with one stone, and Water to run off so we can keep track of those numbers. Of course, this is always open to adjustment... LOL
If this is a water only soil, don't do runoff. If the soil is good you don't need to worry about the pH coming out. The micro organisms will do all the work for you. If you aren't using nutes, you don't need to worry about runoff pH at all. I have NEVER even checked mine, in either KindSoil or my homemade Stank Soil. If you go to runoff, you run the risk of washing away the good guys that you need in your soil to give the plants what they need. The only time I would ever really endorse a going to runoff or flush is if your soil is 2nd or 3rd run and there is a risk of any sort of buildup in your soil. You would want to essentially flush it and then hit with a recharge of the good guys. First run, you don't want to do it.